Proinflammatory cytokines act in the brain to regulate important biobehavioral processes that are critical for physical and mental health. Patients receiving cytokine immunotherapy for treatment of viral infections or malignancies and those suffering from chronic inflammatory diseases are at an increased risk of developing depressive disorders. The neurobiological mechanisms and immune substrates underlying these immune-brain interactions remain unclear. Acute and chronic activation of the peripheral innate immune system in laboratory rodents precipitates depressive-like biobehaviors. New data from our laboratory have demonstrated that these biobehaviors are mediated by cytokine-induced activation of the tryptophan-degrading enzyme indoleamine 2, 3 dioxygenase (IDO), which is the first and rate-limiting enzyme of the kynurenine pathway. We demonstrated that pharmacologic inhibition or genetic deletion of IDO prevents depressive-like biobehaviors following peripheral immune activation. Our most recent data point to the generation of neuroactive kynurenine metabolites in the brain as the likely neurobiological mechanism for increased depressive-like biobehaviors. Clinically, reduced circulating tryptophan and increased kynurenine concentrations (the first catabolic product of IDO activity) are correlated with increased depression scores in patients undergoing cytokine immunotherapy. Our new data extend these clinical observations to show that depressive-like behavior in immune-stressed mice requires cytokine-induced IDO activity. In fact, direct administration of kynurenine induces depressive-like behavior in naive mice, but kynurenine is not neuroactive. Instead, kynurenine is compartmentally metabolized in the brain to generate free radical-producing or glutamatergically-active kynurenine metabolites. Microglia produce the free radical forming metabolites 3-hydroxykynurenine and 3-hydroxyanthranilic acid en route to generating the ionotropic glutamate receptor agonist, quinolinic acid. Conversely, endothelial cells and astrocytes produce kynurenic acid, which is a glutamate and 17-nicotinic acetylcholine receptor antagonist. Based on these data, we hypothesize that kynurenine metabolism in activated microglia drives development of inflammation-induced depressive-like biobehaviors. We will first determine if microglial activation is responsible for IDO-dependent depressive-like behavior following peripheral immune stress. Objective two will define the regulatory pathways that link peripheral immune activation to changes in brain kynurenine metabolism, and objective three will utilize ex vivo and in vitro techniques to specifically define how kynurenine metabolism is regulated at the cellular level. The last objective will use pharmacologic and genetic approaches to target kynurenine pathway enzymes that might alleviate cytokine-induced depressive-like biobehaviors. These preclinical experiments are needed to understand the neurobiological mechanisms by which proinflammatory cytokines cause psychopathology and to identify novel therapeutic targets for the treatment of inflammation-induced depression.